Microorganisms associated with Sporobolus anglicus, an invasive dimethylsulfoniopropionate producing salt marsh plant, are an unrecognized sink for dimethylsulfide

Background

Saltmarshes are hotspots of organosulfur compound cycling due to production of dimethylsulfoniopropionate (DMSP) by benthic microorganisms, macroalgae, and saltmarsh vegetation. Degradation of DMSP is a source of dimethylsulfide (DMS), an important precursor for formation of secondary organic aerosol. Microorganisms degrading DMS play a role in controlling the amount of DMS available for emission into the atmosphere. Previous work has implicated sediment microbial populations as a major sink for DMS. Here, we show that Sporobolus anglicus (previously known as Spartina anglica), a widely distributed saltmarsh plant, is colonized by DMS-degrading microorganisms.

Methods

Dimethylsulfide degradation potential was assessed by gas chromatography and 13C-DMS stable isotope probing, microbial community diversity and functional genetic potential in phyllosphere and rhizosphere samples was assessed by high-throughput sequencing of 16S rRNA gene amplicons, cloning and sequencing of methanethiol oxidase genes, and by metagenomic analysis of phyllosphere microbial communities.

Results

The DMS degradation potential of microbial communities recovered from phyllosphere and rhizosphere samples was similar. Active DMS-degraders were identified by 13C-DMS stable isotope probing and included populations related to Methylophaga and other Piscirickettsiaceae in rhizosphere samples. DMS-degraders in the phyllosphere included Xanthomonadaceae and Halothiobacillaceae. The diversity in sediment samples of the methanethiol oxidase (mtoX) gene, a marker for metabolism of methanethiol during DMS and DMSP degradation, was similar to previously detected saltmarsh mtoX, including those of Methylophaga and Methylococcaeae. Phyllosphere mtoX genes were distinct from sediment mtoX and did not include close relatives of cultivated bacteria. Microbial diversity in the phyllosphere of S. anglicus was distinct compared to those of model plants such as rice, soybean, clover and Arabidopsis and showed a dominance of Gammaproteobacteria rather than Alphaproteobacteria.

Conclusion

The potential for microbial DMS degradation in the phyllosphere and rhizosphere of Sporobolus anglicus suggest that DMS cycling in saltmarshes is more complex than previously recognised and calls for a more detailed assessment of how aboveground activities affect fluxes of DMS.

Retinal topography in two species of flamingo (Phoenicopteriformes: Phoenicopteridae)

In this study, we assessed eye morphology and retinal topography in two flamingo species, the Caribbean flamingo (Phoenicopterus ruber) and the Chilean flamingo (P. chilensis). Eye morphology is similar in both species and cornea size relative to eye size (C:A ratio) is intermediate between those previously reported for diurnal and nocturnal birds. Using stereology and retinal whole mounts, we estimate that the total number of Nissl-stained neurons in the retinal ganglion cell (RGC) layer in the Caribbean and Chilean flamingo is ~1.70 and 1.38 million, respectively. Both species have a well-defined visual streak with a peak neuron density of between 13,000 and 16,000 cells mm^-2 located in a small central area. Neurons in the high-density regions are smaller and more homogeneous compared to those in medium- and low-density regions. Peak anatomical spatial resolving power in both species is approximately 10-11 cycles/deg. En-face images of the fundus in live Caribbean flamingos acquired using spectral domain optical coherence tomography (SD-OCT) revealed a thin, dark band running nasotemporally just dorsal to the pecten, which aligned with the visual streak in the retinal topography maps. Cross-sectional images (B-scans) obtained with SD-OCT showed that this dark band corresponds with an area of retinal thickening compared to adjacent areas. Neither the retinal whole mounts, nor the SD-OCT imaging revealed any evidence of a central fovea in either species. Overall, we suggest that eye morphology and retinal topography in flamingos reflects their cathemeral activity pattern and the physical nature of the habitats in which they live.

Amphibious vision - Optical design model of the hooded merganser eye

A comprehensive schematic eye model of the hooded merganser is introduced for the first time to advance the understanding of amphibious vision. It is comprised of two different configurations, the first one modeling its visual system in air (unaccommodated state) and the second one representing the case where the eye is immersed in water (accommodated state). The model was designed using available data of former studies, image analysis and the implementation of feasible assumptions that serve as starting values. An optimization process incorporating an optical design program is used to vary the starting values with the aim of finding the setup offering the best acuity. The image quality was measured using the root-mean-square radius of the focal spot formed on the retina. The resulting schematic eye model comprises all relevant optical specifications, including aspherical geometrical parameters for cornea and lens, distances between the surfaces, the gradient index distribution of the lens, the retinal specifications and the object distance in both media. It achieves a spot radius of 4.20 μm for the unaccommodated state, which meets the expectations derived by the mean ganglion cell density and comparison with other animals. In contrast, under water the determined spot radius of 11.48 µm indicates an acuity loss. As well as enhancing our understanding of the vision of the hooded merganser, the schematic eye model may also serve as a simulation basis for examing similar animal eyes, such as the cormorant or other fish hunting birds.

Understanding the retinal basis of vision across species

The vertebrate retina first evolved some 500 million years ago in ancestral marine chordates. Since then, the eyes of different species have been tuned to best support their unique visuoecological lifestyles. Visual specializations in eye designs, large-scale inhomogeneities across the retinal surface and local circuit motifs mean that all species' retinas are unique. Computational theories, such as the efficient coding hypothesis, have come a long way towards an explanation of the basic features of retinal organization and function; however, they cannot explain the full extent of retinal diversity within and across species. To build a truly general understanding of vertebrate vision and the retina's computational purpose, it is therefore important to more quantitatively relate different species' retinal functions to their specific natural environments and behavioural requirements. Ultimately, the goal of such efforts should be to build up to a more general theory of vision.

Optic flow cues help explain altitude control over sea in freely flying gulls

For studies of how birds control their altitude, seabirds are of particular interest because they forage offshore where the visual environment can be simply modelled by a flat world textured by waves then generating only ventral visual cues. This study suggests that optic flow, i.e. the rate at which the sea moves across the eye's retina, can explain gulls' altitude control over seas. In particular, a new flight model that includes both energy and optical invariants helps explain the gulls' trajectories during offshore takeoff and cruising flight. A linear mixed model applied to 352 flights from 16 individual lesser black backed gulls (Larus fuscus) revealed a statistically significant optic flow set-point of ca 25° s^-1. Thereafter, an optic flow-based flight model was applied to 18 offshore takeoff flights from nine individual gulls. By introducing an upper limit in climb rate on the elevation dynamics, coupled with an optic flow set-point, the predicted altitude gives an optimized fit factor value of 63% on average (30-83% in range) with respect to the GPS data. We conclude that the optic flow regulation principle helps gulls to adjust their altitude over sea without having to directly measure their current altitude.